You’re fit, you feel great and you run, a lot. So who cares if your abs aren’t as flat as they used to be? Even if your weight’s not on your radar as a health issue, though, it should be as a performance one. Because there’s a good chance you’re not at your ideal racing weight—that is, the weight at which you run your fastest and feel your best.

Perhaps you have always had the same body and never considered what adding a few pounds of muscle or dropping a few pounds of flab might do for your performance. Alicia Shay, a professional runner and nutrition counselor in Flagstaff, Arizona, says weight shouldn’t be overlooked. “Anyone who cares how fast they’re running should consider their weight part of their overall training strategy,” she says.

It’s most common for runners to find their weight has crept up over the years. Pete Magill, author of Build Your Running Body, didn’t think much about his own gradual weight gain until, at 44, his usual 15-minute 5K times began to suffer.

“When I ran 16:20, I knew I was in trouble,” Magill says. “I’d been racing at 170 pounds since starting masters competition, almost 10 pounds over my race weight back in my 20s.” He couldn’t train any harder, so he went on a diet and dropped to 164. Over the next few years, he set the American men’s 45–49 record for the 5K, at 14:34.

Magill’s not alone. “I can’t imagine you can talk to a competitive runner who doesn’t have a weight-loss-equals-faster-time story,” he says.

But getting to that ideal number can be hard work—especially if you’re already logging major mileage and are used to eating whatever you want. And it’s a delicate balance: Dipping below it or losing weight in unhealthy ways could put you at risk for injury, illness and disordered eating behaviors.

Why Lighter Equals Faster

As a general rule, runners move most efficiently when they’re at the low end of what’s considered a healthy body mass and body-fat percentage. “Running is really just a form of jumping,” says Matt Fitzgerald, certified sports nutritionist and author of Racing Weight: How to Get Lean for Peak Performance. “You can’t move forward without moving up, and the more you have to lift against gravity, the more energy it requires.” Imagine running with an extra 5 pounds strapped around your waist, he explains.

Body weight affects performance in running more than it does in other sports, such as swimming and biking, according to a 2011 Swiss study on Ironman triathletes. And lower body-mass index seems increasingly important as race distances get longer: A 2014 study found that the optimal BMI for male 800m runners was between 20 and 21, while it dropped between 19 and 20 for male 10,000m and marathon runners. (Generalizations about BMI shouldn’t be used prescriptively, Shay says, because it doesn’t take into account lean muscle or body fat.)

There are other reasons lighter means faster: Larger people are less efficient at delivering oxygen throughout the body. Losing weight doesn’t change your lung capacity or function, but it does mean that each breath doesn’t have to go as far.

Leaner athletes can dissipate heat better, too, because they have a higher surface-area-to-body-weight ratio and less insulating fat tissue. They also burn carbohydrates more efficiently. So while weight isn’t everything, it’s certainly a big thing. “Four factors determine how fast you are,” says Sean Wade, a top masters athlete and coach of the Houston-based Kenyan Way running program. “Genetics, form, how hard and smart you train, and your weight—and not necessarily in that order.”

Age Makes a Difference

Masters runners may have the hardest time losing weight—no matter how much they run. Fred Zalokar, 54, averages more than 100 miles a week and for years oscillated between 170 and 180 pounds. But since he cleaned up his diet four years ago, he’s dropped more than 20 pounds, increasing his speed and turnover along the way.

“Weight loss has never just happened for me, even when I was regularly running ultras,” Zalokar says. “If I’m not consciously cutting my calories, I can make up for all that mileage without a second thought.”

That’s because adults begin losing muscle mass in their early 40s, which can impair performance and decrease metabolism, says Emily Brown, RD. Getting enough physical activity (including resistance training to maintain muscle) is especially important, as is cutting back on junk food and oversize food portions.

Tom Storey, 50, who has run Boston seven times, attributes his first qualifying run, in 2007, to weight loss. In just more than a year, he went from 205 to 150 pounds and shaved more than 30 minutes off his previously 4-hour marathon time. Today, maintaining his weight requires sacrifices he’s willing to make. “There aren’t a lot of things I can do to make myself a better athlete,” he says, “but if I can keep my weight down, I know I’ll be faster.”

Crunching the Numbers

Finding your ideal weight—the healthy weight at which you really perform your best—takes some time. “You can’t predict your optimal race weight beforehand,” Fitzgerald says. “You can move toward a goal, and when you have the race of your life, you weigh yourself and get your body composition tested, and there you have it.”

Andrew Lemoncello, 2012 Olympian for the U.K. and a coach for McMillan Running, didn’t find his ideal race weight until going pro after college. “I used to live by the saying, ‘If the furnace is hot enough, it will burn anything,’ “ he says. “I ate healthy foods, but I also loved desserts and never paid attention to my portions.”

When he began spending time with other pro runners, Lemoncello realized he needed higher-quality fuel for his furnace to run at its most efficient. He began planning his meals and stopped mindless eating, and he dropped from 150 pounds to 145. “I started setting PRs and had more energy and confidence,” he says. “If I’m eating well and training hard, that’s the weight my body naturally gravitates to.”

For any weight loss or gain, it helps to have something to aim for. Several online calculators, like Fitzgerald’s at RacingWeight.com, will estimate your ideal racing weight based on your age, gender and current fitness level. In this case, ideal is defined as what you would weigh if your body fat was at its lowest attainable-yet-healthy percentage, Fitzgerald explains.

Then there are the stats about how much quicker you’ll be by slimming down, like the commonly cited 2 seconds per mile, per pound you are above your ideal weight. But this will vary from person to person, says exercise physiologist Paul Vanderburgh, creator of the online Flyer Handicap Calculator, which helps runners see how their race times stack up against competitors of other ages and weights. Based on VO2 max estimates, it computes your predicted time if you were 25 years old and a scale model of yourself at 110 pounds for women or 143 pounds for men. “It’s strictly meant for comparisons,” he stresses, “not for figuring out the weight you should realistically be to hit a certain time.”

In Build Your Running Body, Magill and his co-authors plot a chart of estimates based on VO2 calculations—for example, that a 200-pound runner can shave 19 seconds off a 20-minute 5K time by losing 5 pounds. Wade takes a simpler approach: “One minute slower per 1 pound overweight is what I tell my marathoners,” he says.

These tools can be good motivators, according to Rasa Troup, a certified specialist in sports dietetics (CSSD), a 2008 Olympian and current nutritionist for Team USA Minnesota, but she adds that athletes shouldn’t take them as gospel. “My biggest concern is that they distract runners from listening to and understanding their own bodies, because they’re caught up in reaching some number,” she says. “It’s more important to be aware of how tired hungry or sluggish you are feeling.”

Many runners could benefit from shedding a few pounds, Troup admits—but only if they have excess weight to lose and only if it’s done in a way that supplements their training, rather than sabotaging it. “Well-fueled athletes will perform well, and well-fueled means something different for every person,” she says. Of course, losing weight isn’t a good idea for all competitive runners. For those who are naturally very lean or who work hard to stay at the low end of their healthy weight, the threat of falling below that point at which you race your best is real.

Stephanie Bruce learned that the hard way when she finished a disappointing 16th at the 2013 Boston Marathon. She and her husband, pro runner Ben Bruce, have spent years perfecting their race-weight strategies, she says, and weigh themselves daily leading up to a big race.

“We didn’t have a scale in Boston, and it was difficult to get in as many calories the day before as I would have been able to at home,” she says. (Bruce has celiac disease and has to be careful about eating food prepared outside of her own kitchen.) “I was probably under by only 2 or 3 pounds on race morning, but it made a huge impact. I had no power; I just couldn’t hold the pace.”

Bruce says she works hard to maintain a healthy weight and has never dropped so low that her health has suffered. But many runners, without enough calories or nutrients, can develop weak bones and compromised immune systems.

That’s what Brian Rosetti figures happened to him. After he graduated from college, he spent two years training almost full-time. His mileage was increasing, but he was focused on low weight instead of nutrition for performance. Just as he made a breakthrough in his training, and as his weight dropped to an all-time low of 146 on his 6-foot-1 frame, he suffered a sacral stress fracture. “My bone density was below the median level, and I don’t think I was getting the right nutrients,” Rosetti says. “I was focused on keeping as light as I could. That’s a scary place to be.” The injury, in effect, ended his career.

Fitzgerald says impaired performance is usually the first sign that a runner has dipped into dangerous territory. “It’s the canary in the coal mine—your body’s signal that it’s under too much stress.”

For women, a missing menstrual period is also an indication of an unhealthy and unsustainable weight, with potential complications like infertility and osteoporosis. And while it’s less talked about, competitive men can struggle, too. A recent Southern Utah University study found that almost 20 percent of male high school cross country runners were at risk for disordered eating behaviors like bingeing and purging. Some boys expressed a desire to gain body weight to be more attractive, while others wanted to lose it to improve their running.

To keep your weight loss from becoming detrimental, the American Council on Exercise recommends maintaining a BMI that stays at or above the normal weight threshold of 18.5 and a body fat percentage above 14 percent for women and 6 percent for men. Some elites dip below these guidelines, Troup says, but it’s not recommended without careful monitoring.

Fitzgerald also recommends tracking your performance. “If you’re getting skinnier but your times are getting worse, you’ve passed the point of beneficial weight loss,” he says.

And above all else, Shay says, listen to your body and your mind. “If you’re starving all the time or you’re irritable and cranky or you’re bonking on runs, you’re probably being too ambitious and getting too light.”

Timing is Everything

The time to prioritize weight loss is in a four- to nine-week period before you start ramping up your workouts, while you’re building your base. “You can’t maximize fitness gain and weight loss simultaneously,” Fitzgerald says.

Mueller agrees. During a competitive season, athletes shouldn’t restrict themselves by more than 500 calories per day. For those who want to lose only a few pounds, 200 to 300 is even better. Stop restricting calories the week of a big race, she adds, because your body will perform best on a full tank.

Bruce says she goes into her training cycles about 3 to 5 pounds over her racing weight, a product of relaxed eating habits and less exercise during her offseason. “I like to have a reserve to pull from, because once I start working out harder, I lose it pretty easily,” she says.

Lemoncello follows a similar schedule during his training period, gaining 5 to 10 pounds when he’s not racing. “It’s good for my running,” he says. “The break helps me feel energized, and I come back motivated.”

A Part of Hearst Digital Media
Runner's World participates in various affiliate marketing programs, which means we may get paid commissions on editorially chosen products purchased through our links to retailer sites.